There are microspheres and microballoons, the red variety made of Bakelite resin I believe. They don't add strength. They lighten the weight of resin mix by making it a mechanical foam. They also make it easer to sand or work when cured, and are usd to fair a surface, though don't necessarily provide a fine finish when used alone.
They and other fillers also reduce the expense of the resin. Commonly used with epoxy are colloidal silica (heavy and much more difficult to sand, but abrasion resistant as a result), microfibers (generally mlled cotton fibers), and in the 70's when I had my boatshop, milled asbestos fibers, which did add strength supposedly. Also graphite powder, which was primarily used not as a lubricant for a molded bearing (though I did wonder about the possibilities) but as a UV inhibitor. Epoxies are usually quite susceptible to UV degradation.
FRP is fiber reinforced plastic, and that is made up differently than fillers generally, and usually involves saturating a mat, cloth, or unidiectional tow of organic textiles (linen, cotton, etc.) or synthetics (glass, carbon, polypropylene, Dynel, Kevlar, etc.) with a resin (polyester, vinylester, epoxy, etc.) in a matrix.
Fillers and fabrics can both be used in combination, but they have different functions, for the most part. There are some reinforcing fiber fillers, like chopped strand glass, but a FRP product made of those tends to be heavier and have lower mechanical qualities than a laminated cloth or tow version. An FRP boat hull laid up with a chopper gun is weak, heavy, and brittle compared to a proper cloth laminated version.
Decks of small craft have been covered in cotton duck and paint (a resin) for ages, and paper was laminated with varnish into fine canoes well over a century ago -- See Nathaniel Bishop's Voyage in the Paper canoe. I'm sure references to similar processes can be traced back a couple millennia.